A conventional example of microgyroscopes with a gyroscope and a signal processing circuit for processing a signal outputted therefrom, which are integrated in a single semiconductor chip, is disclosed in John A. Geen et al. “Single-chip Surface Micromachined Integrated Gyroscope With 50° C./h Allan Deviation” IEEE Journal of Solid-State Circuits, Vol. 37, No. 12, December 2002.
The microgyroscope disclosed in the document is provided with a charge amplifier configured to convert a charge signal outputted from a gyroscope into an output voltage. The charge amplifier includes an operational amplifier and a feedback resistor with a high resistance of the order of, for example, 500 MΩ; this high resistance is required to stabilize the output voltage of the operational amplifier.
As a feedback resistor with such a high resistance, which is to be installed in a single semiconductor chip as an Integrated Circuit (IC), a MOS resistor, that is, a MOSFET (Field Effect Transistor) with an adjustable high on-resistance, has been used in common. This is because the MOSFET can provide an adjustable high resistance with its compact size.
In the conventional charge amplifier with the structure set forth above for gyroscopes, because of the feedback MOS resistor's high resistance, the period required for the output of the charge amplifier to become stable after power-on thereof may become long. In other words, in the conventional charge amplifier with the structure set forth above, the period required for the charge amplifier to begin to self oscillate after power-on thereof may become long.
Specifically, after power on of the charge amplifier, an input offset component with respect to a reference voltage applied to the non-inverting input terminal of the operational amplifier is amplified thereby to be fed back as a feedback signal to the inverting input terminal of the operational amplifier through the feedback resistor. The feedback signal component is amplified by the operational amplifier to be fed back as a feedback signal to the inverting input terminal of the operational amplifier through the feedback resistor again. The repeated feedback through the feedback resistor allows the charge amplifier to gradually begin to self oscillate with respect to the reference voltage.
In this case, the lower the resistance of the feedback MOS resistor is, the shorter the period required for the charge amplifier to start to self oscillate can be. This MOS resistor's resistance lowering means may however cause the converting characteristic of the charge amplifier to deteriorate. In other words, the MOS resistor's resistance lowering means may cause the output voltage of the charge amplifier to become unstable. This may be because, the lower the resistance of the feedback resistor is, the larger the magnitude of the feedback signal flowing through the feedback resistor is.